Confidence is good. Arrogance is not. The key is to be confident but also open to other people's ideas and to the possibility that you may be mistaken and/or your design could be improved.
Let's use Sherlock Holmes as an example. Sherlock Holmes was the world's first consulting detective, and he was incredibly skilled in deduction and the history of crime. He was very confident in his abilities, and his repeated successes reinforced that confidence. He also was often arrogant, and this character flaw alienated many he worked with, even his "Boswell" Dr. Watson. His arrogance sometimes distracted from his ability to convince people, as it "turned off" the people he needed to believe him.
Now, obviously, we are not all at the super-expert level of a Sherlock Holmes or a Dr. House (his modern-day counterpart), but if we were it would probably be hard to NOT be arrogant… though, as I note below, such arrogance is self-defeating. He needed a partner to keep such arrogance in check – to keep him "grounded."
Confidence comes with experience, and experience is the best teacher. But the key is to know what you DON'T know, and to ask questions whenever they need to be asked.
Finally, I'm a very passionate advocate of "peer designing," where two designers work together on a problem. Not only does it double the brainpower that is being applied, but it also allows for easy peer reviews and design critique. Much like Holmes needed Watson, I think we work best when we work with a partner to keep us in check.